SkSVGCanvas and co. depend on expat.

SkSVGCanvas depends on SkSVGDevice which depends on SkXMLWriter.cpp
which depends on expat.  So don't build any of this if we don't have
expat.

Change-Id: I1ffca2d58d3b607febf11ce75abdd6efe08f49c8
Reviewed-on: https://skia-review.googlesource.com/8289
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Mike Klein <mtklein@chromium.org>
diff --git a/BUILD.gn b/BUILD.gn
index f4221b9..1f95d60 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -537,6 +537,8 @@
     "//third_party/expat",
   ]
   sources = [
+    "src/svg/SkSVGCanvas.cpp",
+    "src/svg/SkSVGDevice.cpp",
     "src/xml/SkDOM.cpp",
     "src/xml/SkXMLParser.cpp",
     "src/xml/SkXMLWriter.cpp",
@@ -608,8 +610,6 @@
     "src/ports/SkOSFile_stdio.cpp",
     "src/sfnt/SkOTTable_name.cpp",
     "src/sfnt/SkOTUtils.cpp",
-    "src/svg/SkSVGCanvas.cpp",
-    "src/svg/SkSVGDevice.cpp",
     "src/utils/mac/SkStream_mac.cpp",
     "third_party/etc1/etc1.cpp",
     "third_party/gif/SkGifImageReader.cpp",